The present invention relates generally to room temperature voltage tunable varactors and tunable devices that include such varactors.
Phased array antennas are comprised of a large number of elements that emit phase controlled signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. This electronic beam steering concept applies to both transmitters and receivers. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to their speed, accuracy, and reliability. The replacement of gimbal scanned antennas by their electronically scanned counterpart can provide more rapid and accurate target identification. Complex tracking exercises can also be performed rapidly and accurately with a phased array antenna system.
Adjustable phase shifters are used to steer the beam in phased array antennas. Previous patents in this area include ferroelectric phase shifters in U.S. Pat. Nos.: 5,307,033, 5,032,805, and 5,561,407. These phase shifters include one or more microstrip lines on a ferroelectric substrate as the phase modulate elements. The permittivity of the ferroelectric substrate may be varied by varying the strength of an electric field on the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the microstrip line. The microstrip ferroelectric phase shifters disclosed in those patents suffer high conductor losses and impedance matching problems due to the high dielectric constant of the ferroelectric substrates.
Future communications will employ wideband frequency-hopping techniques, so that large amount of digital data can be transferred over the band. A critical component for these applications is a low cost fast-acting tunable filter. Digital data could be distributed or encoded over a band of frequencies in a sequence determined by controlling circuitry of the tunable filter. This would allow several users to transmit and receive over a common range of frequencies.
Varactors can be used independently utilized or can be integrated into low cost tunable filters. These varactors and filters can be used at numerous frequency ranges, including frequencies above L-band, in a myriad of commercial and military applications. These applications include (a) L-band (1-2 GHz) tunable filters for wireless local area network systems, personal communications systems, and satellite communication systems, (b) C-band (4-6 GHz) varactors and tunable filter for frequency hopping for satellites communications and radar systems (c) X-band (9-12 GHz) varactors and filters for use in radar systems (d) Ku band (12-18 GHz) for use in satellite television systems, and (e) KA band tunable filters for satellites communications.
Common varactors used today are Silicon and GaAs based diodes. The performance of these varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range and figure of merit, or Q factor (1/tan xcex4) at the specified frequency range. The Q factors for these semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly. In fact, at 10 GHz the Q factors for these varactors are usually only about 30.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film ferroelectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film ferroelectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
There is a need for varactors that can operate at temperatures above those necessary for superconduction and at frequencies up to 10 GHz and beyond, while maintaining high Q factors. In addition, there is a need for microwave devices that include such varactors.
A voltage tunable dielectric varactor includes a substrate having a first dielectric constant and having generally planar surface, a tunable ferroelectric layer positioned on the generally planar surface of the substrate, with the tunable ferroelectric layer having a second dielectric constant greater than the first dielectric constant, and first and second electrodes positioned on a surface of the tunable ferroelectric layer opposite the generally planar surface of the substrate. The first and second electrodes are separated to form a gap therebetween. A bias voltage applied to the electrodes changes the capacitance of the varactor between an input and an output thereof.
The invention also encompasses phase shifters that include the above varactors. One embodiment of such phase shifters includes a rat race coupler having an RF input and an RF output, first and second microstrips positioned on the rat race coupler, a first reflective termination positioned adjacent to an end of the first microstrip, and a second reflective termination positioned adjacent to an end of the second microstrip, wherein the first and second reflective terminations each includes one of the tunable varactors.
Another embodiment of such phase shifters includes a microstrip having an RF input and an RF output, first and second radial stubs extending from the microstrip, a first varactor positioned within the first radial stub, and a second varactor positioned within the second radial stub, wherein each of the first and second varactors is one of the above tunable varactors.
The planar ferroelectric varactors of the present invention can be used to produce a phase shift in various microwave devices, and in other devices such as tunable filters. The devices herein are unique in design and exhibit low insertion loss even at frequencies greater than 10 GHz. The devices utilize low loss tunable bulk or film dielectric elements.